I’m speculating that the reasons would be:
- Reusable launchers make this an economic reality. It becomes just affordable enough for a super-rich clientele.
- That broadens the marketplace for rocket services, from companies and government who want to launch satellites (minuscule market), to multi-millionaires who want to show off (larger market).
- That means increased revenue for SpaceX, with diversified income streams, which enables faster investment in Mars colonisation.
- It also means more frequent launches, and the need for more diversity of launch vehicles, which means more frequent opportunities to experiment and optimise the product line… Leading to better, cheaper rockets… and a virtuous cycle ensues.
In other words, the more business SpaceX does, the better it becomes at rocketry, and the faster it can get humans to Mars in large numbers, and support their early colonisation efforts. It’s the same gambit as with Tesla’s Roadster and Model S/X leading to the Model 3: Make a product for the rich to subsidize the development of a platform for the mass market.